1. Field of the Invention
The present invention relates to an electrophotographic photosensitive material, and more particularly to an improvement in an electrophotographic photosensitive material using amorphous silicon as a photoconductive material.
2. Description of prior arts
Heretofore, as photoconductive materials used for formation of the photoconductive layer of electrophotographic sensitive material, there are known inorganic materials, such as amorphous selenium, selenium alloy, metallic compound semiconductors (for example, oxides, sulfides, selenides, etc. of cadmium, zinc, etc.); organic polymers such as polyvinyl carbazole, etc.; and organic compounds such as colorant, pigment, etc. Recently, there has been proposed an electrophotography using photoconductive amorphous silicon for the formation of the photoconductive layer of the electrophotographic photosensitive material.
As methods for forming the photoconductive layer of the electrophotographic photosensitive material composed of amorphous silicon, there are known, for example, a method wherein a powdery amorphous silicon is dispersed in a binder to form a photoconductive layer, and a method wherein a silicon-containing compound such as a silane or a silane derivative in the vapor form is decomposed by glow discharge and caused to be superposed on an electrically conductive support. Generally, these materials comprise basically an electrically conductive support and a photoconductive layer composed of amorphous silicon containing a silicon atom and a hydrogen atom and/or a halogen atom provided on said support.
However, such electrophotographic photosensitive materials comprising only an electrically conductive support and an amorphous silicon photoconductive layer have certain disadvantages so that there have been proposed improved methods, for example, a method wherein an intermediate layer is provided between the electrically conductive support and the amorphous silicon photoconductive layer, and a method wherein another layer is provided on the amorphous silicon photoconductive layer.
For example, the electrophotographic photosensitive material comprising only the electrically conductive support and the amorphous silicon photoconductive layer has a disadvantage in that the material is easily influenced by ambient temperature and humidity, so that the characteristics as the photosensitive material are liable to fluctuate, and further that the surface of the amorphous silicon photoconductive layer is easily oxidized by the corona charging (particularly negative-charging) to form an insulating oxidized film having high resistance on the surface of said photoconductive layer. If the photoconductive layer having the insulating oxidized film formed on the surface thereof is used for the electrophotographic process, there occurs lowering in the resolution of the formed image, i.e. formation of fog. Such lowering in resolution is thought to arise from the fact that when the corona discharge is carried out in the form of the image on the surface of the photoconductive layer having the insulating oxidized film formed on the surface thereon, a phenomenon that electric charges laterally migrate on the interface between the oxidized film and the photoconductive layer under the film.
In order to improve such drawback of the photosensitive material wherein the surface of the photosensitive layer is exposed to the outside, there is proposed in Japanese Patent Provisional Publication No. 57(1982)115559 a photoconductive member wherein a surface barrier layer containing a large amount of carbon atom is provided on the upper side of said photoconductive layer. In this photoconductive member, the surface barrier layer protects the photoconductive layer and inhibits the migration of electric charge on the charged surface into the photoconductive layer.
When the above-described surface barrier layer is provided on the surface of the photoconductive layer, it becomes possible to prevent the surface of said layer from oxidation, but when an electrophotographic process is carried out by using such photosensitive material composed of the amorphous silicon photoconductive layer having the surface barrier layer provided thereon, the surface barrier layer shows property of inhibiting the migration of electric charge. Accordingly, there occurs a problem in that the resolution of the formed image can not reach a satisfactory level from the initial stage of the electrophotographic process.